The overall aim of this project is to gain a more complete understanding of the factors contributing to the maintenance of venous return, and therefore cardiac filling pressure, in humans in conditions of circulatory and thermal strain. To this end we will use exercise in the heat as our experimental tool and superimpose hypohydration as an additional load. We will monitor cardiac output, forearm blood flow and circulating blood volume and relate changes in these variables to the body temperatures. We will examine these relations in fit and unfit individuals, and over the course of a physical training regime, in an attempt to understand the physiological mechanisms that can be called upon to maintain cardiac filling pressure, and thereby resist heat syncope and other related disorders. We will also examine the effects of separate modifications of blood volume, blood osmolality and blood (Na ion) on these relations. This should provide insight into the interrelation between the circulatory and temperature regulatory systems. Hypovolemia and hypervolemia will be induced by several different techniques, in order to change volume with and without changes in other blood constituents. Finally an important goal that has long-term significance for this project is the development of a working animal model. This model would permit our conducting parallel studies between humans and dogs and give us the ability to ask questions that cannot be asked with human subjects. For instance one goal of our studies with the animal model will be to study the effects of microinjection of arginine vasopressin into the preoptic anterior hypothalamus on circulatory and temperature regulation during exercise in the heat. With these approaches we will have the capability to become both more mechanistic in our approach and yet retain the applicability derived from our human studies.